Compression apparatus for gaseous refrigerant

ABSTRACT

Apparatus ( 30 ) for compressing gaseous refrigerant for use in a refrigeration circuit ( 2 ) of a liquefaction plant, which refrigeration circuit ( 2 ) has an inlet ( 5 ), a first outlet ( 6 ) for refrigerant at low pressure, a second outlet ( 7 ) for refrigerant at intermediate pressure, a third outlet ( 8 ) for refrigerant at high pressure and a fourth outlet ( 9 ) for refrigerant at high-high pressure, which apparatus ( 30 ) comprises a first and a second compressor ( 31   a   , 31   b ), wherein the first compressor ( 31   a ) has a main inlet ( 36 ) connected to the first outlet ( 6 ), a side-inlet ( 37 ) connected to the third outlet ( 8 ) and an outlet ( 38 ) connected to the inlet ( 5 ) of the refrigeration circuit ( 2 ), and wherein the second compressor ( 31   b ) has a main inlet ( 39 ) connected to the second outlet ( 7 ), a side-inlet ( 40 ) connected to the fourth outlet ( 9 ) and an outlet ( 41 ) connected to the inlet ( 5 ) of the refrigeration circuit ( 2 ).

[0001] The present invention relates to an apparatus for compressinggaseous refrigerant for use in a refrigeration circuit of a liquefactionplant.

[0002] U.S. Pat. No. 4,698,080 discloses a liquefaction plant of theso-called cascade type having three refrigeration circuits operatingwith different refrigerants, propane, ethylene and methane. In the firsttwo of these refrigeration circuits the natural gas is pre-cooled, andin the third refrigeration circuit the natural gas is liquefied.

[0003] In the first two refrigeration circuits, the propane circuit andthe ethylene circuit, the refrigerant is compressed in an apparatus forcompressing gaseous refrigerant to a refrigeration pressure and suppliedto three heat exchangers in series, wherein in each heat exchanger therefrigerant is allowed to evaporate at a lower pressure in order toremove heat from the natural gas feed. The refrigerant is allowed topartly evaporate in the first heat exchanger at high pressure. Thevapour part of the refrigerant at high pressure leaving the first heatexchanger is returned to the compression apparatus and the remainingliquid is allowed to partly evaporate at intermediate pressure in thesecond heat exchanger. The vapour part of the refrigerant atintermediate pressure leaving the second heat exchanger is returned tothe compression apparatus and the remaining liquid is allowed toevaporate at low pressure in the third heat exchanger. The refrigerantat low pressure leaving the third heat exchanger is returned to thecompression apparatus.

[0004] The third refrigeration circuit, the methane circuit, differsfrom the other two. A difference is that the natural gas that has beenpre-cooled at liquefaction pressure is liquefied in a main heatexchanger by indirect heat exchange with natural gas. The natural gasused for liquefaction is obtained downstream of the main heat exchanger.Downstream of the main heat exchanger, the pressure of the liquefiednatural gas is let down in three stages in order to enable storingliquefied natural gas at atmospheric pressure. The three stages yieldthree streams of gaseous natural gas. The three streams of natural gasused for liquefying the natural gas are compressed in a compressionapparatus to liquefaction pressure and returned to the natural gas feedupstream of the main heat exchanger.

[0005] The compression apparatus used in the propane circuit is a singlecompressor comprising three sections. The compressor has a main inlet,two side inlets and one outlet for refrigerant at refrigerationpressure. The main inlet is the inlet for refrigerant at low pressure,the first side inlet is the inlet for refrigerant at intermediatepressure and the second side inlet is the inlet for refrigerant at highpressure.

[0006] The compression apparatus used in the ethylene circuit comprisestwo compressors in series, a first compressor having two sections and asecond compressor having one section. The first compressor has a maininlet, a side inlet and one outlet for refrigerant at high pressure,wherein the main inlet is the inlet for refrigerant at low pressure andthe side inlet is the inlet for refrigerant at intermediate pressure.The second compressor, having only one section, has a main inlet forrefrigerant at high pressure and an outlet for refrigerant atrefrigeration pressure. The first and second compressor areinterconnected.

[0007] The compression apparatus used in the methane circuit comprisesthree compressors in series, wherein each compressor consists of asingle section.

[0008] An alternative to the cascade-type liquefaction plant is theso-called propane-precooled multicomponent refrigerant liquefactionplant. Such a plant has a multi-stage propane pre-cooling circuit thatis of the kind as described above with reference to the first tworefrigerant circuits. In stead of propane, the multi-componentrefrigerant can be pre-cooled by multicomponent refrigerant. An exampleof such a plant is disclosed in U.S. Pat. No. 5,832,745. The apparatusfor compressing the multi-component refrigerant is also a three-sectioncompressor.

[0009] The amount of cooling provided per unit of time in therefrigeration circuit is proportional to the mass flow rate of therefrigerant that is circulated through the refrigeration circuit. Withincreasing amounts of natural gas to be liquefied the mass flow rate ofthe refrigerant has to increase. Although an increasing mass flow ratedoes not affect the number of impellers, it has an effect on the size ofthe impellers, on the diameter of the housing, and on the inlet velocityinto the impellers. Because the latter variables increase withincreasing flow rate, an increasing flow rate will result in a largercompressor and higher inlet velocities. Moreover, increasing thediameter of the housing of the compressor requires a thicker wall of thehousing. Consequently the compressor is more difficult to manufactureand more difficult to handle.

[0010] It is an object of the present invention to provide an apparatusfor compressing gaseous refrigerant that overcomes this drawback.

[0011] To this end the present invention provides an apparatus forcompressing gaseous refrigerant for use in a refrigeration circuit of aliquefaction plant, which refrigeration circuit has an inlet forrefrigerant at a refrigeration pressure, a first outlet for gaseousrefrigerant at a low pressure, a second outlet for gaseous refrigerantat an intermediate pressure and a third outlet for gaseous refrigerantat a high pressure, which apparatus comprises according to the presentinvention a first compressor and a second compressor, wherein the firstcompressor has a main inlet for receiving the refrigerant from the firstoutlet, a side inlet for receiving the refrigerant from the third outletand an outlet that can be connected to the inlet of the refrigerationcircuit, and wherein the second compressor has a main inlet forreceiving the refrigerant from the second outlet and an outlet that canbe connected to the inlet of the refrigeration circuit.

[0012] The problems relating to the compressor size are even morepronounced with more recent liquefaction plants where the refrigerant isallowed to evaporate in four heat exchangers in series.

[0013] For this reason the invention further relates to an apparatus forcompressing gaseous refrigerant for use in a refrigeration circuit of aliquefaction plant, which refrigeration circuit has an inlet forrefrigerant at a refrigeration pressure, a first outlet for gaseousrefrigerant at a low pressure, a second outlet for gaseous refrigerantat an intermediate pressure, a third outlet for gaseous refrigerant at ahigh pressure and a fourth outlet for gaseous refrigerant at a high-highpressure, which apparatus comprises according to the present invention afirst compressor and a second compressor, wherein the first compressorhas a main inlet for receiving the refrigerant from the first outlet, aside-inlet for receiving the refrigerant from the third outlet and anoutlet that can be connected to the inlet of the refrigeration circuit,and wherein the second compressor has a main inlet for receiving therefrigerant from the second outlet, a side-inlet for receiving therefrigerant from the fourth outlet and an outlet that can be connectedto the inlet of the refrigeration circuit.

[0014] The invention will now be described by way of example in moredetail with reference to the accompanying drawings, wherein

[0015]FIG. 1 shows a schematically a refrigeration circuit including aconventional compressor having four sections; and

[0016]FIG. 2 shows schematically a refrigeration circuit including thecompression apparatus according to the present invention having foursections.

[0017] Reference is made to FIG. 1 showing schematically a compressor 1for use in a refrigeration circuit represented by a box 2. Since therefrigeration circuit is well known, it is here only schematically shownfor the sake of clarity.

[0018] The refrigeration circuit 2 has an inlet 5 for refrigerant at arefrigeration pressure, a first outlet 6 for gaseous refrigerant at alow pressure, a second outlet 7 for gaseous refrigerant at anintermediate pressure, a third outlet 8 for gaseous refrigerant at ahigh pressure and a fourth outlet 9 for gaseous refrigerant at ahigh-high pressure.

[0019] The compressor 1 has four sections 10, 11, 12 and 13 arranged ina single housing, which sections are interconnected. Each section cancomprise one or more impellers, wherein an impeller is sometimesreferred to as a stage. The compressor 1 has a main inlet 15, three sideinlets 16, 17 and 18, and an outlet 19. The main inlet 15 opens into thelow pressure section 10, the first side inlet 16 opens into theintermediate pressure section 11, the second side inlet 17 into the highpressure section 12, and the third side inlet 18 into the high-highpressure section 13. For the sake of clarity the driver of thecompressor is not shown.

[0020] The outlet 19 of the compressor 1 is connected to the inlet 5 ofthe refrigeration circuit 2 by means of conduit 20. The first outlet 6of the refrigeration circuit 2 is connected to the main inlet 15 of thecompressor 1 by means of conduit 21, the second outlet 7 is connected tothe first side inlet 16 by means of conduit 22, the third outlet 8 isconnected to the second side inlet 17 by means of conduit 23 and thefourth outlet 9 is connected to the third side inlet 18 by means ofconduit 24.

[0021] During normal operation, the compressor 1 compresses therefrigerant to a refrigeration pressure, wherein the refrigerationpressure is the pressure at which the refrigerant is supplied viaconduit 20 to the inlet 5 of the refrigeration circuit 2. In four heatexchangers (not shown) in series the refrigerant is allowed toevaporate. In the first heat exchanger the refrigerant is allowed topartly evaporate at a high-high pressure, which is below therefrigeration pressure; the liquid part of the refrigerant is passed tothe second heat exchanger and the remaining vapour (D kg/s) is returnedto the compressor 1 through conduit 24. In the second heat exchanger therefrigerant is allowed to partly evaporate at a high pressure, which isbelow the high-high pressure; the liquid part of the refrigerant ispassed to the third heat exchanger and the remaining vapour (C kg/s) isreturned to the compressor 1 through conduit 23. In the third heatexchanger the refrigerant is allowed to partly evaporate at anintermediate pressure, which is below the high pressure; the liquid partof the refrigerant is passed to the forth heat exchanger and theremaining vapour (B kg/s) is returned to the compressor 1 throughconduit 22. In the forth heat exchanger the refrigerant is allowed toevaporate at a low pressure, which is below the intermediate pressure,and the refrigerant leaving the forth heat exchanger (A kg/s) isreturned to the compressor 1 through conduit 21.

[0022] In the low pressure section 10, A kg/s of refrigerant iscompressed to the intermediate pressure. In the intermediate pressuresection 11, A+B kg/s of refrigerant is compressed to the high pressure.In the high pressure section 12, A+B+C kg/s of refrigerant is compressedto the high-high pressure. In the high-high pressure section 13, A+B+C+Dkg/s of refrigerant is compressed to the refrigeration pressure.

[0023] Reference is now made to FIG. 2 showing schematically anapparatus 30 for compressing gaseous refrigerant according to thepresent invention for use in a refrigeration circuit. The refrigerationcircuit and its inlet and outlets have been given the same referencenumerals as in FIG. 1.

[0024] The apparatus 30 for compressing gaseous refrigerant comprises afirst compressor 31 a and a second compressor 31 b, each compressor 31 aand 31 b being arranged in a single housing. The first compressor 31 ahas two interconnected sections 32 and 33, and the second compressor 31b has two interconnected sections 34 and 35. Each section can compriseone or more impellers. The sections 32, 33, 34 and 35 are referred to asthe low pressure sections 32 and 34 and the high pressure sections 33and 35.

[0025] The first compressor 31 a has a main inlet 36, a side inlet 37,and an outlet 38. The second compressor 31 b has a main inlet 39, a sideinlet 40 and an outlet 41. The main inlet 36 of the first compressor 31a opens into the low pressure section 32, and the side inlet 37 opensinto the high pressure section 33. The main inlet 39 of the secondcompressor 31 b opens into the low pressure section 34, and the sideinlet 40 opens into the high pressure section 35. For the sake ofclarity the drivers of the compressors are not shown.

[0026] The outlets 38 and 41 of the compressors 31 a and 31 b areconnected to the inlet 5 of the refrigeration circuit 2 by means ofconduits 50, 50 a and 50 b. The first outlet 6 of the refrigerationcircuit 2 is connected to the main inlet 36 of the first compressor 31 aby means of conduit 51, and the second outlet 7 is connected to the maininlet 39 of the second compressor 31 b by means of conduit 52. The thirdoutlet 8 is connected to side inlet 37 of the first compressor 31 a bymeans of conduit 53, and the fourth outlet 9 is connected to the sideinlet 40 of the second compressor 31 b by means of conduit 54.

[0027] During normal operation, the two compressors 31 a and 31 b eachcompress a part of the refrigerant to the refrigeration pressure, sothat all refrigerant is supplied at the refrigeration pressure viaconduits 50, 50 a and 50 b to the inlet 5 of the refrigeration circuit2. In four heat exchangers (not shown) in series the refrigerant isallowed to evaporate. In the first heat exchanger the refrigerant isallowed to partly evaporate at a high-high pressure, which is below therefrigeration pressure; the liquid part of the refrigerant is passed tothe second heat exchanger and the remaining vapour (D kg/s) is returnedto the second compressor 31 b through conduit 54. In the second heatexchanger the refrigerant is allowed to partly evaporate at a highpressure, which is below the high-high pressure; the liquid part of therefrigerant is passed to the third heat exchanger and the remainingvapour (C kg/s) is returned to the first compressor 31 a through conduit53. In the third heat exchanger the refrigerant is allowed to partlyevaporate at an intermediate pressure, which is below the high pressure;the liquid part of the refrigerant is passed to the forth heat exchangerand the remaining vapour (B kg/s) is returned to the second compressor31 b through conduit 52. In the forth heat exchanger the refrigerant isallowed to evaporate at a low pressure, which is below the intermediatepressure, and the refrigerant leaving the forth heat exchanger (A kg/s)is returned to the first compressor 31 a through conduit 51.

[0028] In the low pressure section 32 of the first compressor 31 a, Akg/s of refrigerant is compressed to the high pressure, and in the highpressure section 33, A+C kg/s of refrigerant is compressed to therefrigeration pressure. In the low pressure section 34 of the secondcompressor 31 b, B kg/s of refrigerant is compressed to the high-highpressure, and in the high pressure section 35, B+D kg/s of refrigerantis compressed to the refrigeration pressure.

[0029] A comparison between the compressors discussed with reference toFIGS. 1 and 2 shows that that the low pressure section 10 of compressor1 corresponds to the low pressure section 32 of the first compressor 31a, and that the high-high pressure section 13 corresponds to the highpressure section 35 of the second compressor 31 b. However, because ofthe different line-up, the intermediate pressure section 11 correspondsto the low pressure section 34 of the second compressor 31 b, and thehigh pressure section 12 corresponds to the high pressure section 33 ofthe first compressor 31 a.

[0030] The differences in mass flow rates in the conventionalfour-section compressor and the apparatus for compressing gaseousrefrigerant according to the present invention will now be summarized inthe below Table. TABLE Differences in mass flow rate through thesections of the compressors. Conventional Section compressor Inventionlow pressure A A intermediate pressure A + B B high pressure A + B + CA + C high-high pressure A + B + C + D B + D

[0031] An advantage of the compression apparatus according to thepresent invention is that in the three sections following the lowpressure section the mass flow rates are smaller. Consequently thevolumetric flow rates in these sections are smaller.

[0032] In case the refrigeration circuit only includes three heatexchangers, the compression apparatus comprises three sections. Two ofthe three sections are arranged in the first compressor and the secondcompressor is the third section. In that case the line-up is like theone shown in FIG. 2 except that conduit 54 is not present, and thatthere is no high pressure section 35.

[0033] The compressors in the apparatus according to the presentinvention are suitably axial compressors.

1. Apparatus for compressing gaseous refrigerant for use in arefrigeration circuit of a liquefaction plant, which refrigerationcircuit has an inlet for refrigerant at a refrigeration pressure, afirst outlet for refrigerant at a low pressure, a second outlet forrefrigerant at an intermediate pressure and a third outlet forrefrigerant at a high pressure, which apparatus comprises a firstcompressor and a second compressor, wherein the first compressor has amain inlet for receiving the refrigerant from the first outlet, a sideinlet for receiving the refrigerant from the third outlet and an outletthat can be connected to the inlet of the refrigeration circuit, andwherein the second compressor has a main inlet for receiving therefrigerant from the second outlet and an outlet that can be connectedto the inlet of the refrigeration circuit.
 2. Apparatus for compressinggaseous refrigerant for use in a refrigeration circuit of a liquefactionplant, which refrigeration circuit has an inlet for refrigerant at arefrigeration pressure, a first outlet for refrigerant at a lowpressure, a second outlet for refrigerant at an intermediate pressure, athird outlet for refrigerant at a high pressure and a fourth outlet forrefrigerant at a high-high pressure, which apparatus comprises a firstcompressor and a second compressor, wherein the first compressor has amain inlet for receiving the refrigerant from the first outlet, aside-inlet for receiving the refrigerant from the third outlet and anoutlet that can be connected to the inlet of the refrigeration circuit,and wherein the second compressor has a main inlet for receiving therefrigerant from the second outlet, a side-inlet for receiving therefrigerant from the fourth outlet and an outlet that can be connectedto the inlet of the refrigeration circuit.